1. Field of the Invention
The present invention relates to an optical fiber inclinometer, and more particularly, to a sensor that uses fiber Bragg grating devices and thereby serves to measure the skew or inclined angle of a structure.
2. Description of the Related Art
The two basic elements of an optical fiber are its core and cladding closely covering the core. The refractive index of the core is greater than that of the cladding; therefore, light traveling inside an optical fiber is always confined to the core because total internal reflection occurs whenever light travels from a high-density medium (high refractive index area) to a low-density medium (low refractive index area). As a result, light may be transmitted a long distance in the high-density medium.
In 1987, K. O. Hill created a fiber grating, which was the first of its kind, in germanium-doped core, using argon-ion laser. Not only are fiber gratings widely applied to optical fiber communications system, but also they are commonly used in the field of measurement. In 1989, Meltz and others exposed photosensitive optical fibers to high-energy ultraviolet laser, to alter their molecular bonding and thereby increase their refractive index. Since there is periodic variation in the refractive index of the optical fibers along axial directions, this device is also known as fiber Bragg grating (FBG).
FIG. 1 is a perspective diagram of an optical fiber 10 having fiber Bragg gratings. The optical fiber 10 contains a core 13 of total length L, and coverings over the core 13 are a cladding 12 and a protective layer 11 in sequence. An incident ray 14 enters the core 13 at the left end and exits from the right end to have a transmitted ray 15. Owing to a regular periodic variation of refractive index along the axis of the optical fiber 10, the incident ray 14 of a specific wavelength cannot pass the core 13 and is reflected and returns to the original point of incidence (the left end).
FIG. 2(a) is a diagram showing wavelength distribution of the incident ray and reflected ray in FIG. 1. The incident ray 14 comprises light of a certain broad range of wavelengths, whereas the reflected ray 16 comprises light of a specific wavelength λb1 which belongs to the fixed narrow range of wavelengths, thus the light of wavelength λb1 is missing from the transmitted ray 15. The wavelength λb1 is called the Bragg wavelength, as shown in FIG. 2(b).
If the optical fiber 10 is subjected to temperature variation or an external force and thereby causing an extension ΔL in the axial direction, the Bragg wavelength shifts from λb1 to λb2, as shown in FIG. 3. Compression may otherwise occur, making the Bragg wavelength shifts from λb2 to λb3. Hence, the following equation is obtained.
                    λ                  b          ⁢                                          ⁢          2                    -              λ                  b          ⁢                                          ⁢          1                            λ              b        ⁢                                  ⁢        1              =                    K        t            ×      Δ      ⁢                          ⁢      T        +                  K        e            ×      ɛ      
Where ΔT denotes temperature difference, Kt denotes temperature sensitivity coefficient, Ke denotes strain sensitivity coefficient, and ε denotes axial strain, or the quotient of ΔL divided by L.
If axial strain equals 10−6 at a constant temperature, then Bragg wavelength drift Δλ=λb2−λb1 ranges between 0.00115 and 0.0012 nm. Since fiber Bragg grating devices may function as high-precision sensors for measuring physical variables like strain and temperature, they are widely applied to the monitoring of stress and deformation in civil structures. Unlike conventional resistive-type strain gauges which have drawbacks, such as multiple and complex cables required for each individual measurement points, and being susceptible to electromagnetic interference and susceptibility to adverse environment—humidity and high temperature for example, fiber Bragg gratings have a number of advantages, such as energy saving, long distance signal transmission, broad transmission bands, being adverse environment resistant, and, more importantly, multiple point and concurrent measurements of strain, using optical fibers characterized by single-line series connection. Therefore, fiber Bragg gratings are excellent alternatives to conventional resistive-type strain gauges on various applications, such as inclinometers or tiltmeters.